Increasing the density of integrated circuits can increase speed and enable new applications. The increased density can increase undesirable electrical interactions between adjacent circuit elements and conducting lines. Unwanted interactions are typically prevented by providing trenches that are filled with electrically insulating material to isolate the elements both physically and electrically. As circuit densities increase, however, the widths of these trenches decrease, increasing their aspect ratios and making it progressively more difficult to fill the trenches without leaving voids. A trench which is not completely filled is undesirable since the degree of isolation may be compromised which limiting the maximum operational frequency or otherwise adversely affect operation of the integrated circuit.
Common techniques that are used in gapfill applications are chemical-vapor deposition (“CVD”) techniques. Conventional thermal CVD processes supply reactive gases to the substrate surface where thermally-induced chemical reactions take place to produce a desired film. Plasma-enhanced CVD (“PECVD”) techniques promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (“RF”) energy to a reaction zone near the substrate surface creating a plasma. The high reactivity of the species in the plasma reduces any thermal energy required to promote a chemical reaction thereby lowering the temperature required for such CVD processes when compared with conventional thermal CVD processes. These advantages may be further exploited by high-density-plasma (“HDP”) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that ionized reactants form a greater portion of the total reactant population. The mean free path can be extended in HDP-CVD for the same density of ionized reactants and their impingement velocities can be increased in magnitude and directionally controlled. While each of these techniques falls broadly under the umbrella of “CVD techniques,” each of them has characteristic properties that make them more or less suitable for certain specific applications.
Inclusion of boron in films grown with CVD techniques can lower the dielectric constant relative to silicon nitride, improving the electrical isolation provided by a trench filled with a boron-containing film. The dielectric constant may be reduced by an amount which varies based upon the amount of boron included, other elements present in the film and the particular deposition techniques employed. The presence of boron can also decrease the etch rate and cause other changes in film properties which may be desirable for a given step in a semiconductor manufacturing process sequence.
Especially for front-end processes, the presence of boron in a deposited film introduces a risk of migration of boron into the substrate. Boron is a common dopant used to affect the performance of the active region of a silicon transistor. The addition of boron may change a lightly doped region to a more heavily doped region and negatively affect the performance of a device. Therefore it is desirable to develop techniques for inhibiting the diffusion of boron into underlying layers.